An important factor in marketing and purchasing produce is uniformity in size and quality. Food producers typically find it more economical to pack and ship a uniform grade of produce, and consumers expect a certain grade of produce for a given price. To accomplish the grading operation, a food crop is typically sorted by size after the crop is harvested. Automation of this process has been somewhat successful, as many machines have been developed to grade fruits and vegetables by size. One particular machine design shakes the produce along sloped diverging rods. When the distance between the rods exceeds the size of the produce, the produce falls between the rods. The distance the produce travels along the rods before falling indicates the size of the produce. Sorting machines of this type are preferred because of the simplicity of the design.
However, such sorting machines have several limitations. Sloped rods may cause the produce to roll along the rods and bruise adjacent produce. If the rods are horizontal, a vertical component has been added to the movement of the rods so that the produce will move along the diverging rods. This is also undesirable because vertical shaking can also cause bruising. In addition, known graders shake the produce by employing a rotating eccentric shaft or cam mechanism to impart periodic oscillation to the rods. Since the amplitude and acceleration of the oscillatory motion are functions of the shape of the eccentric or cam, the only way to vary the shaking motion of the produce is to vary the speed of the periodic oscillation. Too low of a speed limits the amount of produce that can be sorted by the grader in a given time, and too high of a speed risks damaging the produce.